High pH in water after being in container?

[KingBlingTX]

I just got a pH measurement of 8.64 in my saltwater that I mixed up a few days ago. It has been stored in a plastic 5 gal jug with the lid on it. The pH of a fresh mix is generally around 8.2.

I understand pH swings during the day/night cycle in a tank due to CO2 fluctuations caused by photosynthetic activity, but why would the pH rise so high in the storage container?

 

[SROPE]

Ph meter calibration perhaps?

 

[KingBlingTX]

I don't think so. It measures correct values in my aquariums.

 

[Randy Holmes-Farley Verified Community Expert]

Not sure, but perhaps some extra salt mix solids are still slowly dissolving. Or something was left in the containers.

It is true that only CO2 and alkalinity will be impacting the pH, if accurate.